Typically a colour image is formed by superimposition of a number of digitally specified separations or printing colour images.
In a method of this kind, a colour image is represented by pixels arranged in a rectangular raster, the values of the pixels specifying a colour in the form of a co-ordinate value in a colour space.
In the production of a print from a colour image of this kind, digital image data are first calculated which (for each pixel) specify (in a colour space) a co-ordinate value adapted to the ink colours of a printing unit, hereinafter referred to as the printing colours. This coordinate value specifies the degree of coverage for each of the printing colours, conventionally expressed in eight binary bits. Many colour printing units work with the printing colours yellow, magenta, cyan and black (Y, M, C, K) but there are also printing units which contain more printing colours, usually the colours already mentioned and in addition red, blue and green (R, B, G). A partial image formed by the values of the pixels of a separate printing colour is usually referred to as a separation.
The digital image data thus calculated are then used to control a printing unit. The different separations are successively converted to an ink image and fixed in combination on an image support, usually a sheet of paper, whereafter they jointly form a multi-colour image by optical mixing of the printing colours.
In the calculation of the digital image data, in the first instance there is calculated for each separation and for each pixel therein a value which specifies a degree of coverage and hence an optical density in eight binary bits, i.e. 256 possible values. The most usual printing units, such as electrophotographic printers or ink jet printers, however, can only process two pixel values, namely “ink” and “no ink” or 0 and 1. The 8-bit pixel values should therefore be converted to binary pixel values which can be processed by a printing unit.
Various techniques are available to convert multi-value pixels into binary pixel values without losing a shaded overall impression of the printed image. These techniques all make use of the integrating power of the human eye whereby images built up of a sufficient number of small dots are perceived by the observer as a uniform surface. These techniques are generally referred to by the collective name of “half-tone processing”. Known techniques are dithering, in which regular patterns of black and white pixels are printed, and thresholding, in which only pixels having a relatively high value are actually printed. The latter technique is frequently supplemented by error diffusion, in which rounding-off errors are passed on to pixels which are still to be treated.
The dithering technique is very suitable for reproducing uniform surfaces, but less suitable for sharp edges because of its inherently low resolution.
The thresholding technique gives good reproduction of sharp transitions and even emphasises them to some extent, but uniform surfaces having a degree of coverage between maximum and minimum values are forced towards one of two extreme values without shading.
The combined technique of thresholding and error diffusion is very suitable for sharp transitions. Uniform surfaces are also reasonably reproduced thereby although frequently with some noise. However, it does not approach the quality level of dithering.
The said techniques are described in detail in the literature and therefore require no further explanation.
For good reproduction of all kinds of image information it is therefore desirable to be able to use both techniques, depending on the local image type. This is described, for example, in U.S. Pat. No. 4,930,007 to Sugiura et al.
In the method known from this patent, a colour image is divided into small blocks of pixels, and of each block the predominant type of image is determined from the K-signal (the “black” signal; this signal usually defines the image content most strongly). In blocks where an edge predominates thresholding is used for all the separations while in the other blocks dithering is used for all the separations. In this way, edges in the image are printed with the technique which can best reproduce the sharp transitions so that the image quality is improved.
In this method, therefore, all the separations are treated in the same way, i.e. with the same half-tone technique. However, this can lead to unwanted effects. This will be explained by an example.
At a colour transition at least one of the separations will frequently extend over the transition. For example, if a blue surface (formed by superimposition of magenta and cyan) adjoins a red surface (formed by magenta and yellow), then the cyan and the yellow separations will each contain a sharp edge and the magenta separation of one surface will extend into the other, possibly with different degrees of coverage. The cyan separation and the yellow separation, which dominate the colour transition, then benefit from the edge-strengthening effect of thresholding, but for the magenta separation, which has no or practically no transition and hence more of a surface characteristic thresholding is precisely the wrong choice, because thresholding is less suitable for surface reproduction. Since the adjoining pixel blocks are treated with dithering which is the optimum for surfaces, there is introduced into the magenta image a density transition which does not correspond to the original image and therefore has a negative effect on the print quality.